1. Field of the Invention
The present invention pertains to the field of processing biomass to produce fuels, chemicals and other useful products and, more specifically, to hydrolyzing cellulosic biomass to produce sugars for conversion to ethanol and other products. Use of a prewash to remove minerals before acidifying the biomass results in significantly improved acid pretreatment efficiency.
2. Description of the Related Art
Cellulosic biomass represents an inexpensive and readily available source of sugars. These sugars may be used alone, fermented to produce alcohols and industrial chemicals, or chemically converted to other compounds. For example, cellulosic biomass is useful for generating ethanol, which has a number of industrial and fuel uses. Of particular interest is the use of ethanol as a gasoline additive that boosts octane, reduces pollution, and partially replaces gasoline in fuel mixtures.
Generally speaking, biomass, e.g., wood, grass, forest or crop residue, contains cellulose wrapped in a recalcitrant lignin and hemicellulose sheath. The sheath must be chemically and/or physically disrupted in a pretreatment step that produces some sugars and provides access to cellulose. Typical pretreatment protocols involve mechanical size reduction, acid hydrolysis, ammonia or alkali treatment, and/or steam explosion. High capital and operating costs are associated with all of these pretreatment methods. For example, most of the pretreatments are carried out at high temperatures and a considerable amount of energy is used to heat the biomass. These high temperatures create the evolution of steam and other gaseous products which create high pressures, with concomitantly high containment costs. Further, the elevated pressures make it difficult to introduce solid materials into the reactor.
In acid pretreatments, nitric or hydrochloric acid may be used, but sulfuric acid is often favored because of its lower cost. However, pretreatment expenditures may still be large when sulfuric acid is used because substantial quantities of acid are required, and neutralization and disposal costs remain. It has been found by A. Esteghlalian, A. G. Hashimoto, J. J. Fenske, and M. H. Penner, Bioresour. Technol., 59, 1997, 129-136 and J. N. BeMiller, Adv. Carbohydr. Chem., 22, 1967, 25-108, that cellulosic biomass can have a significant mineral content, and that these minerals neutralize some of the acid during dilute-acid pretreatment, which increases acid demand. For example, mineral oxides combine with sulfuric acid to form sulfate salts and water:Nm+2Om+mH2SO4 Nm+2(SO4)m+mH2O  (1)where N is potassium, sodium, calcium, iron and other cations, and m is an integer equal to the charge of the cation. This neutralization reaction consumes hydrogen ions in the formation of water.
Specific to sulfuric acid is an equilibrium shift to formation of bisulfate that can further reduce hydrogen ion concentrations and compound the effect of neutralization, as reported by J. M Readnour and J. W. Cobble, Inorg. Chem., 8(10), 1969, 2174-2182. Equation (2) shows the sulfate salt formed in Equation (1) combining with hydrogen ions to form a bisulfate salt:Nm+2(SO4)m+mH+ Nm+(HSO4)m+Nm+  (2)
This equilibrium shift has a more pronounced effect at lower acid concentrations, where the quantity of hydrogen ion consumed in Equation (2) represents a large percentage of the total hydrogen ion concentration. Further, bisulfate salt formation is favored when dilute solutions are subjected to the high temperatures used for pretreatment reactions because the equilibrium constant, K2=([H+][SO42−])/[HSO4−], decreases as temperature increases. Due to both bisulfate salt formation and a shift in the sulfuric acid dissociation equilibrium, an increase in pH is observed and additional acid is required to achieve a particular reaction rate.